Wireless communications systems enable users of User Equipment (UE) to communicate with other such users via one of a number of base stations and a core network. Typically, the UEs are mobile terminals, such as cellular telephones and the like. In an active or connected state a UE is registered with the network and has a Radio Resource Control (RRC) connection with a base station so that the network knows to which base station (or cell thereof) the UE belongs and can transmit data to and receive data from the UE. The UE also establishes a default Evolved Packet System (EPS) Bearer to an endpoint beyond the base station, typically a gateway, in the Enhanced Packet Core (EPC) network, or core network for short. An EPS Bearer (which also includes Radio Bearer (RB) element between each UE and their respective serving base station) defines a transmission path through the network and assigns an IP address to the UE, at which it can be reached by other communication devices, such as another UE. An EPS Bearer also has a set of data transmission characteristics, such as quality of service, data rate and flow control parameters, which are defined by the subscription associated with the UE and are established by the Mobility Management Entity (MME) upon registration of the UE with the network.
The EPS Bearer is thus managed by the MME, which signals to the UE when it needs to activate, modify, or deactivate a particular EPS Bearer. The Radio Bearer is managed by the base station (which might further be controlled by the MME). Thus there are always two connections between the UE and the communication network: one for the user data transmitted using the established EPS bearer (also known as the user plane) and another one for managing the EPS Bearer and its elements (also known as the control plane). Further details of the EPS Bearer architecture can be found in 3GPP TS 23.401 V11.2.0 and 3GPP TS 36.300 V11.2.0, the contents of which are hereby incorporated by reference.
As part of the Long Term Evolution (LTE) Of UTRAN (UMTS Terrestrial Radio Access Network) referred to as E-UTRAN, there are plans to introduce a feature of direct device-to-device (D2D) communication when the UE can communicate user data to another UE that is within the transmission range of the first UE without the need to use LTE network resources. This direct communication would result in better utilization of the available resources, especially on the radio interface, where these are limited.
D2D services might be particularly beneficial for users of professional mobile radio (PMR) networks, such as Terrestrial Trunked Radio (TETRA), and the like. Since PMR networks are mainly used by government agencies, emergency services, (police threes, fire departments, ambulance), rail transportation staff, transport services and the military, D2D voice and/or data services may be used:                to ensure increased resilience in case of network failure by using direct mode as a fall-back solution;        to support reliable communication between mobile telephones in areas where network coverage is normally not required/provided; and        to create person-to-person or person-to-group communications in areas without network coverage at all, such as large buildings, tunnels, basements, etc.        
Although in LTE the D2D communication is predominantly controlled by the network operator (i.e. the setting up of direct radio bearers between two user devices is executed based on control parameters received from their respective serving base stations), in some cases (e.g. for users of PMR) it might be beneficial to allow user devices to initiate D2D communication with each other without any network provided control parameter. This may be beneficial (or even necessary) in some of the above mentioned cases, especially when a D2D communication session involves at least one user device that is operating outside the coverage area of LTE base stations and hence would not able to obtain D2D control information from the network. Therefore, if such D2D capable user devices are within communication distance from each other, they might set up a D2D communication channel between them without the LTE network provided control parameters.
However, when user devices are operating a D2D channel that has been set up without control parameters provided by the network, a particular problem might arise that their transmissions over this D2D channel might cause harmful interference to nearby LTE devices (and vice versa). Such interference is even more likely to happen when the D2D channel (at least partially) overlaps with the LTE channels (i.e. the licensed LTE spectrum) used by the network or when these user devices (re)enter the coverage area of an LTE base station whilst still transmitting on the D2D channel.